Dielectronic Capture with Subsequent Two-Electron Emission in Electron-Impact Ionization of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>Ions

In electron-impact ionization of ${\mathrm{C}}^{3+}$ ions we found the first unambiguous experimental evidence for resonant-excitation auto-double-ionization processes. These involve capture of the projectile electron by the ion with simultaneous $K$-shell excitation and subsequent emission of two correlated electrons. The resonance strength measured for the $1s2{s}^{2}2p^{3}P$ contribution is (2.0 \ifmmode\pm\else\textpm\fi{} 0.4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}20}$${\mathrm{cm}}^{2}$ eV. In addition, we resolved individual thresholds of excitation autoionization via states $1s2s2l$ and identified other resonant contributions to net single ionization.